During the production of integrated circuits, semiconductor devices, magnetic disks, optical disks and the like, free contaminant particles are usually present in the processing equipment and in the environment in which the constituents being processed are disposed. The contaminant particles that are found, particularly in the gases and liquids employed in the processing equipment, adversely affect the circuits, devices and assemblies being produced Therefore it has been found necessary to detect or sense the number and density of particles so that the process operator can determine whether the magnitude of contamination is greater than a predetermined threshold of particle density.
To accomplish the desired detection, sensor technology that monitors the flow or flux of particles in free space is utilized. The requirements of a sensor for monitoring particles are that it can operate in a vacuum, that it is compact enough to fit into existing equipment, that it does not contaminate the vacuum, and that it provides real-time data. Also, if possible the sensor should be able to tolerate the harsh environments, which may contain free fluorine or chlorine radicals, for example, that may be present in the processing equipment. In addition, because the gas flows that are formed in and around such processing equipment are frequently turbulent and carry small particles, it is desirable to be able to sense the direction of motion of the particles.
When operating with detection instruments that determine the number of free particles in gases and liquids, it is assumed that the particulate is suspended in the gas or liquid. The gas or liquid acts as a carrier to transport the suspended particles through the focus of a laser beam that is used with the detection equipment. The suspended particles scatter light that enables detection and thereby provide an indication of the size of the particles.
However, the sensor technology that has been employed is characterized by severe limitations when used in vacuum equipment. For example, it is not feasible to employ a gas as a carrier that will bring the particles to the focus point of a laser beam. Also, a measurement technique using a carrier gas is not passive because the carrier gas is drawn from the measurement point. Therefore, the gas flow is affected and environmental conditions at the point of measurement are affected, particularly when measurements are done in small volume vessels. Since there is a time delay between the time the gas enters the tube which is used to draw the carrier gas to the laser, and the time when the gas passes the laser beam, it is apparent that such a measurement does not occur in real time. The results obtained with such equipment are difficult to correlate to actual events at the point of measurement. Furthermore, the carrier gas measurement relies on the assumption that the particles remain in suspension. But it is known that heavier particles are not suspended to enable proper measurement and therefore the heavier particles will not be detected. In addition, such measurements provide no information about the direction of particle motion, since the drawing of carrier gas is an isotropic process.
It would be highly advantageous to provide a particle detection apparatus wherein a high intensity response to detected particles, including small particles, is obtained, and wherein the area of the response region, over which the response as a function of particle size is above the background noise level, is significantly extended.